1. Field of the Invention
This invention relates to a detection circuit for detecting the ratio of the intensities of the color components of light, and particularly relates to such a detection circuit for use in an automatic color temperature adjusting device for a video camera.
2. Description of the Prior Art
Video cameras are known which include an automatic color temperature adjusting device wherein light from a large area of a scene to be photographed is sensed through an optical system independent of the camera objective lens to measure the intensity or the spectral energy of the blue and red components of the light. The color temperature for the picture being taken is automatically adjusted in accordance with the measurements i.e. the measured results.
An example of such video cameras is shown in FIG. 1. With reference to FIG. 1, the video camera comprises an objective lens 2, a singular type pickup tube 4, a color-brightness separation circuit 6, a gain adjustable amplifier 8 for the red component, a gain adjustable amplifier 10 for the blue component, and an encoder 12. When the electric signal corresponding to the image of the scene formed by objective lens 2 is produced by pickup tube 4, the electric signal is separated into red, blue and brightness components by the color-brightness separation circuit 6, with the electric signal of each component being output from each output terminal 6a, 6b and 6c of the separation circuit 6. The red and blue component signals output from the output terminals 6a and 6b, respectively, will be input to encoder 12 via gain adjustable amplifier 8 for the red component and gain adjustable amplifier 10 for blue component respectively, while the brightness component signal 6c output from output terminal 6c will be directly input to encoder 12, which in turn converts the above component signals into video signals.
The automatic color temperature adjusting device includes filter 14 which transmits the blue component of the scene light; a filter 16 which transmits the red component of the scene light; photodetectors 18 and 20 which receive the lights passing through filters 14 and 16 and output electrical signals proportional to the intensities thereof; logarithmic compression circuits 22 and 24 for logarithmically compressing the respective electric signals; a subtracting circuit 26 for subtracting one from the other of the signals fed from the respective logarithmic compression circuits 22 and 24; and a logarithmic expansion circuit 28 for logarithmically expanding the output from subtraction circuit 26.
It should be noted that photodetectors 18 and 20 are arranged to receive the light from a larger area of a scene to be photographed than objective lens 2.
Assuming that the levels of the output signals from photodetectors 18 and 20 are EB and ER respectively, logarithmic compression circuits 22 and 24 will output signals corresponding to log EB and log ER, respectively, and subtraction circuit 26 will output a signal corresponding to ##EQU1## This means that the logarithmic expansion circuit 28 outputs the signal corresponding to EB/ER thereby providing a signal corresponding to the ratio between the blue and red components. If each gain at the gain adjustable amplifier 10 for the blue component and gain adjustable amplifier 8 for the red component is adjustable, a sufficiently balanced color component signal can be output to encoder 12.
However, since this video camera makes use of such a rule of a logarithmic calculation to obtain the ratio between two color components that subtraction between the respective logarithms of two values are equal to the logarithm of the ratio between the two values, the camera requires a circuit for the logarithmic compression of the electric signals corresponding to the intensities of two color components, a circuit which performs subtraction between the signals which have been logarithmically compressed and a circuit which performs logarithmic expansion of the signal obtained as the result of the subtraction. Thus, the circuit configuration becomes expensively complicated.